In a recent engine (e.g. diesel engine), in order to reduce the emission and improve the fuel-efficiency, the Mass Air Flow (MAF) and Manifold Air Pressure (MAP) are controlled optimally by an intake gas control system.
Typically, as illustrated in FIG. 1, the intake gas control system of the diesel engine includes a MAP control system and MAF control system, and MAP and MAF are independently controlled each other. In order to reduce Particulate Matters (PM) in the exhaust gas, the MAP control system controls a nozzle diameter of a Variable Nozzle Turbo (VNT) to control the MAP. On the other hand, in order to reduce nitrogen oxides (NOx) in the exhaust gas, the MAF control system controls a degree of the valve aperture of an Exhaust Gas Recirculator (EGR) that recirculates the exhaust gas into a cylinder to control MAF. Optimum target values of MAP and MAF, which were experimentally determined according to driving conditions (e.g. injection quantity, engine speed), are outputted to these control systems, and a reference value of the VNT nozzle aperture degree and a reference value of the EGR valve aperture degree are also outputted as feedforward values to the aforementioned control systems by a planning unit. Thus, conventionally, the MAP control system and MAF control system independently operate, and this is called “Single Input Single Output” (SISO). In the SISO control, when the MAP control system and MAF control system interfere with each other, it becomes difficult to simultaneously trace the targets of the MAP and MAF.
Thus, as illustrated in FIG. 2, a cooperative control system is considered in which interference compensators (i.e. first and second interference compensators) compensate the interference of these two control systems. Such a control system is called “Multi-Input Multi-Output” (MIMO) cooperative control system. In this cooperative control system, the control input by the first interference compensator from the MAP control system to the MAF control system and the control input by the second interference compensator from the MAF control system to the MAP control system compensate the interference between both of the control systems in the engine. By introducing this MIMO cooperative control system (hereinafter, which is abbreviated “MIMO control system”), it is said that it is possible to improve the target follow-up characteristic.
Incidentally, there is a technique for the EGR control system, in which, when the EGR valve is opened from an entirely closed state, chattering and/or overshoot is suppressed by adding a slope, hysteresis or time-delay to the target opening degree in the open control. In addition, a technique for stabilizing the control by setting the hysteresis when switching the feedback control target between the high pressure EGR and the low pressure EGR exists in an internal combustion engine using both of the high pressure EGR and the low pressure EGR.
The reduction of the emission is strongly requested from a viewpoint of the environment problem, and a control method is proposed that the completely-closed state of the EGR and VNT is positively utilized in the planning unit. However, the inventor of this application newly found that the emission, especially NOx, increases when the MIMO control system utilizing such a completely-closed state of the EGR and VNT is adopted.